PROJECT SUMMARY and ABSTRACT The clinical outcome of radiation therapy depends on delivering the highest possible absorbed dose to the tumor(s) while limiting the dose to normal tissues. Unfortunately, unlike external beam radiotherapy (EBRT), current RT prescription methods do not consider absorbed dose to individual patients but instead use empirically derived or standard methods. Thus, up to 50% of these patients receive sub-optimal prescriptions leading to sub-optimal clinical outcomes including under-dosing of the tumor or severe toxicity in healthy tissues. Clearly this violates the basic tenants of radiation oncology because the radiation doses received by these patients are neither justified nor optimized. Our solution, RAPID (Radionuclide Assessment Platform for Internal Dosimetry), will be the first commercialized desktop Monte Carlo radionuclide dosimetry system. It will utilize a heterogeneous CPU/GPU (Graphical Processing Units) architecture that will minimize the computational complexity stemming from problems encountered in RT dosimetry. RAPID will provide accurate radiation dosimetry results within a clinically acceptable timeframe of less than 5 minutes. The long-term objective of this project is to develop the first FDA approved patient-specific treatment planning software for RT that is completely accessible. Health care systems, drug companies, and clinical researchers are expected to access this technology to provide the best possible care for cancer patients.